This invention relates to a method of depositing a thick dielectric film, such a silica film, of optical quality on a substrate.
The manufacture of optical devices, such as optical Multiplexers (Mux) and Demultiplexers (DMux), using silica wave guides, requires the deposition of high performance silica films over a silicon wafer. Because of the stringent requirements on the different characteristics of these films, it is important to have very good control (within wafer uniformity, wafer to wafer and run to run reproducibility) over the required physico-chemical and mechanical characteristics.
The most important characteristics to control are the film composition, refractive index, mechanical stress and optical absorption. In addition, the film has to be free of any particles or other contamination. Such particles can act as light absorption sites or introduce defects in the functionality of the device.
The deposition of a thick dielectric layer for optical and micro mechanical devices can be performed using the LPCVD (Low Pressure Chemical Vapor Deposition) technique. This technique deposits the dielectric layer through the introduction of reactive gases into a low-pressure quartz furnace. This reaction is activated by heat only, and the uniformity and reproducibility of the layer depends on different parameters, such as film buildup on the wall of the furnace, temperature profile, quartz ware and gas dynamics in the furnace, which are difficult to control. This technique produces a film with a large number of small size contaminants. These contaminants increase in size with the thickness of the film.
The deposition can also be carried out using the APCVD (Atmospheric Pressure Chemical Vapor Deposition) technique. In this technique, the layer is deposited by injection of reactive gases from a dispersion head to the heated substrate. The deposition of a thick layer in such equipment requires multiple separate depositions and it is difficult to control the reproducibility of the film from one deposition to another. Also, the contamination level of such a deposition technique is high because of the direct contact between the reactive gases and the atmosphere.
In accordance with the principles of the invention, PECVD (Plasma Enhanced Chemical Vapor Deposition) in a commercially available reactor is used for the thick film deposition.
PECVD deposition is normally used for integrated circuit (IC) fabrication where the thickness of films involved is of the order of one micron or less. The deposition of thicker layers is difficult especially for thickness of more than five microns. At these thicknesses, the top electrodes that also act as the gas dispersion head experience overheating by the plasma energy, and the presence of silane (SiH4) in the reactor will cause a silica film buildup on the electrode. This buildup modifies the relative impedance of the plasma and causes a shift in the plasma characteristic and thus in the film characteristics itself. Also, because of the poor adhesion of this silica buildup on the top electrode at an uncontrolled temperature, flakes peel off the surface and fall on the wafer thus creating major contamination. In a sequential deposition reactor, this contamination from the top electrode appears after approximately 200 seconds of continuous deposition under a single showerhead. This limitation on time corresponds to a film of approximately five microns for a deposition rate of 0.215 micron/min that is typical for this process. The overheating of the showerhead can also occur with a thinner film if it is deposited on a large number of wafers. The consecutive deposition of film thicker than one micron will cause overheating that will generate a shift in the film characteristics and contamination. It would thus not be expected that PECVD might be suitable for making optical quality thick films.
The manufacture of optical devices such as optical Mux and DMux using silica wave-guides requires film thickness of 5 to 12 micron for a single layer. The total thickness normally used for a buffer/core/cladding structure is 20 microns or more. At these thicknesses it is not normally possible to deposit films in a single run to obtain the film quality and control needed by integrated optical devices without excessive contamination caused by the top electrode overheating in the reactor.
The present invention permits the production of film of thickness of 20 microns or more processed in a single run for a large number of wafers in the same batch using a special deposition sequence in a commercial reactor.
According to the present invention there is provided a method of depositing thick dielectric films on a substrate, comprising building up said dielectric film by depositing a plurality of layers by PECVD (Plasma Enhanced Chemical Vapor Deposition) in a reactor, each layer having a final thickness less than the thickness of said film to be deposited; and cleaning said reactor between the deposition of each layer.
Using this method the applicants have found unexpectedly that they can deposit dielectric films up to 28 microns or more in a commercially available PECVD reactor without contamination. Such films can be used as high performance optical material.
For example, the multiple step process can be used to deposit a thick layer of silicon dioxide (SiO2) in a manner such as the refractive index, the composition, the mechanical stress are controlled and the contamination of the film is low. This low contamination level leads to minimal light losses when this film is used in a light-propagating device, such as an integrated xe2x80x98Mux/DMuxxe2x80x99 on silicon.
Preferably, a cleaning sequence for the reactor is employed that removes film buildup on the gas dispersion head by a combination of high and low pressure plasma clean which is dependant on the thickness of the film to be deposited and the number of wafers to be processed. A key aspect of the invention is the provision of a cleaning sequence that can be made dependent on the maximum number of wafers to be processed for a particular thickness before a plasma clean is required. The cleaning sequence is carried out automatically and the deposition sequence continues in the same run to guarantee good reproducibility from wafer to wafer.
Typically, the high pressure clean is carried out at about 2.7 Torr, and the low pressure clean at about 0.7 Torr.